Evolution of water balance in the genus Drosophila

Fruit flies of the genus Drosophila have independently invaded deserts arou nd the world on numerous occasions. To understand the physiological mechani sms allowing these small organisms to survive and thrive in arid environmen ts, we performed a phylogenetic analysis of water balance in Drosophila spe cies from different habitats. Desert (cactophilic) species were more resist ant to desiccation than mesic ones. This resistance could be accomplished i n three ways: by increasing the amount of water in the body, by reducing ra tes of water loss or by tolerating the loss of a greater percentage of body water (dehydration tolerance). Cactophilic Drosophila lost water less rapi dly and appeared to be more tolerant of low water content, although males a ctually contained less water than their mesic congeners, However, when the phylogenetic relationships between the species were taken into account, gre ater dehydration tolerance was not correlated with increased desiccation re sistance. Therefore, only one of the three expected adaptive mechanisms, lo wer rates of water loss, has actually evolved in desert Drosophila, and the other apparently adaptive difference between arid and mesic species (incre ased dehydration tolerance) instead reflects phylogenetic history.